Black Colored Steel Sheet Having Excellent Electromagnetic Shielding Property, Electromagnetic Shielding Member, and Electromagnetic Shielding Case

ABSTRACT

A black colored steel sheet comprises a Zn—Ni-plated steel sheet having a blackening-treated layer and a coating film on the surface of the Zn—Ni-plated steel sheet. The blackening-treated layer has a thickness of 0.01 to 1.0 μm. The Zn—Ni-plated steel sheet having a blackening-treated layer has an arithmetical mean roughness Ra of 0.7 to 2.0 μm and the number of peaks per inch (PPI) of at least 180.

TECHNICAL FIELD

The present invention relates to a black colored steel sheet havingexcellent electromagnetic shielding property. In particular, the presentinvention intends to advantageously improve the electromagneticshielding property of a black colored steel sheet, not to mention ablack appearance and corrosion resistance.

The present invention also relates to an electromagnetic shieldingmember and an electromagnetic shielding case, which use the blackcolored steel sheet having electromagnetic shielding property.

BACKGROUND ARTS

Conventionally, black colored steel sheets having a blackening-treatedsurface have widely been used, for example, in business machines, suchas personal computers and copying machines, household electricalappliances, such as air conditioners, automobile parts, and interiorbuilding materials. In general, the black colored steel sheets aremanufactured by applying a black paint to a galvanized sheet iron or byperforming a blackening treatment (for example, anodic electrolysis,cathodic electrolysis, alternating electrolysis, or anodic oxidation) ona plated surface of a Zn—Ni-plated steel sheet and forming at least onecoating film thereon. In the latter method, the coating film is formedbecause of insufficient corrosion resistance of a blackening-treatedsheet.

However, in the former black colored steel sheet, the thickness of thecoating must be increased to completely hide the underlayer with theblack paint. This causes a problem in terms of electromagnetic shieldingproperty.

In the latter black colored steel sheet, an organic coating film and/oran inorganic coating film is formed on the blackening-treated layer.This also causes a problem in terms of electromagnetic shieldingproperty.

However, in the latter black colored steel sheet, when the coating filmis formed on the blackening-treated layer, use of a paint compositioncontaining a metal ion, a water-soluble organic resin, awater-dispersible organic resin, a glycoluril resin, and an acid canreduce the film thickness, improve the electromagnetic shieldingproperty, and provide an excellent black appearance and high corrosionresistance, in spite of a reduced thickness (for example, PatentDocument 1: Japanese Unexamined Patent Application Publication No.2004-188976).

In the surface-treated steel sheet having an organic film and/or aninorganic film, an appropriate combination of the center-line averageroughness Ra and the thickness of the film can provide a surface-treatedsteel sheet exhibiting excellent electromagnetic shielding property andhigh corrosion resistance (for example, Patent Document 2: JapaneseUnexamined Patent Application Publication No. 2004-156081).

In recent years, the problem of electromagnetic interference (EMI) hasbecome obvious in electronic equipment and electrical equipment. Anunnecessary electromagnetic wave leaking from one piece of equipmentcauses functional interference or malfunction in another piece ofequipment. Furthermore, the electromagnetic wave may harm human bodies.

As a countermeasure, a source of noise may be surrounded by a metalplate (conductor). However, an electromagnetic wave may leak from a seamor a joint of a case that surrounds the source of noise. A case formedof surface-treated steel sheets must therefore exhibit sufficientelectromagnetic shielding property at a seam or a joint. Thus, a largenumber of conductive regions must be formed over the entire contactsurface of surface-treated steel sheets that are in contact with eachother at a seam or a joint.

Patent Document 1 proposes a black colored steel sheet exhibiting anexcellent black appearance, high corrosion resistance, and excellentelectromagnetic shielding property. In Patent Document 1, a coating filmformed on a blackening-treated Zn—Ni-plated steel sheet has a reducedthickness to improve the electromagnetic shielding property.

However, Patent Document 1 does not take the surface roughness of theblackening-treated Zn—Ni-plated steel sheet into consideration. When thesurface roughness changes, therefore, it is difficult to form aconductive region at a seam or a joint, or the number of conductiveregions decreases, even when the same coating weight of coating film isdeposited. Hence, the electromagnetic shielding property deteriorates.Furthermore, Patent Document 1 also does not define the thickness of theblackening-treated layer. An increase in the thickness results in thedeterioration of the electromagnetic shielding property.

In a method proposed in Patent Document 2, an appropriate combination ofthe center-line average roughness Ra and the film thickness of thecoating film provides a surface-treated steel sheet exhibiting excellentelectromagnetic shielding property and high corrosion resistance.However, this method does not consider bumps and dips on a surface of asteel sheet, that is, the number of peaks per inch (PPI). A smaller PPIalso results in a decrease in the number of conductive regions at a seamor a joint, leading to poor electromagnetic shielding property.

DISCLOSURE OF THE INVENTION

The present invention solves the problems described above. It is anobject of the present invention to provide a surface-treated steel sheetthat can effectively reduce an electromagnetic wave leaking from a seamor a joint of a case formed of the surface-treated steel sheets toexhibit excellent electromagnetic shielding property and that has anexcellent black appearance and high corrosion resistance.

It is another object of the present invention to provide anelectromagnetic shielding member formed of the black colored steel sheetand an electromagnetic shielding case formed of the black colored steelsheets.

Heretofore, the surface roughness of a Zn—Ni-plated steel sheet having ablackening-treated layer has been defined on the basis of a commonroughness, such as an arithmetical mean roughness Ra, to improve theelectromagnetic shielding property. However, as a result of diligent andrepeated investigations to solve the problems described above, thepresent inventors have found that it is insufficient to define thesurface roughness of a steel sheet on the basis of a common roughness,such as an arithmetical mean roughness, to effectively improve theelectromagnetic shielding property, and that the number of peaks perinch (PPI) is important. The present inventors also have found that thethickness of a blackening-treated layer also has an effect on theelectromagnetic shielding property.

The present invention is based on these findings.

The present invention provides the following:

(1) A black colored steel sheet having an excellent electromagneticshielding property, comprising a Zn—Ni-plated steel sheet having ablackening-treated layer; and a coating film formed on the Zn—Ni-platedsteel sheet, wherein

the blackening-treated layer has a thickness of 0.01 to 1.0 μm;

the Zn—Ni-plated steel sheet having the blackening-treated layer has anarithmetical mean roughness Ra of 0.7 to 2.0 μm and the number of peaksper inch (PPI) of at least 180.

(2) The black colored steel sheet according to (1), wherein the numberof peaks per inch (PPI) is 200 to 400.

(3) The black colored steel sheet according to (1) or (2), wherein thearithmetical mean roughness Ra, the number of peaks per inch (PPI), anda coating weight of coating film applied to one side of the blackcolored steel sheet satisfy the equation (1):z≦(0.010x−0.0077)y−1.05x+2.16  (1)

wherein x denotes the arithmetical mean roughness Ra (μm), y denotes thenumber of peaks per inch (PPI), and Z denotes the coating weight ofcoating film applied to one side of the black colored steel sheet(g/m²).

(4) An electromagnetic shielding member, wherein all or part of themember is formed of the black colored steel sheet according to any oneof (1) to (3); and

(5) An electromagnetic shielding case formed of the black colored steelsheet according to any one of (1) to (3).

The present invention can provide a black colored steel sheet that caneffectively reduce an electromagnetic wave leaking from a seam or ajoint of a case formed of the surface-treated steel sheets to exhibitexcellent electromagnetic shielding property and that has an excellentblack appearance and high corrosion resistance.

The present invention can also provide an excellent electromagneticshielding member all or part of which is formed of the black coloredsteel sheet.

The present invention can also provide an excellent electromagneticshielding case formed of the black colored steel sheets.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating an optimum range of the PPI (y)and the coating weight of coating film z (g/m²) when the Ra (x) is 2.0μm and the PPI (y) is in the range of 200 to 400.

FIG. 2 is a schematic view of an apparatus for measuring leak noise toevaluate the electromagnetic shielding property.

FIG. 3 is a schematic view of a case to evaluate the electromagneticshielding property.

FIG. 4 is a chart illustrating the relationship between the electricfield strength of leak noise and the frequency in a sample ofZn—Ni-plated steel sheet having no coating film (Reference Example).

FIG. 5 is a chart illustrating the relationship between the electricfield strength of leak noise and the frequency in a sample according toExample 7.

FIG. 6 is a chart illustrating the relationship between the electricfield strength of leak noise and the frequency in an open state where nosample is mounted on an aluminum case in the apparatus illustrated inFIG. 2.

FIG. 7 is a chart illustrating the relationship between the electricfield strength of leak noise and the frequency when extraneous noise ismeasured in the absence of an electromagnetic wave and in an open statewhere no sample is mounted on an aluminum case in the apparatusillustrated in FIG. 2.

EMBODIMENT FOR CARRYING OUT THE INVENTION

The present invention will specifically be described below.

The following is a reason for limiting the thickness of ablackening-treated layer formed on a Zn—Ni-plated steel sheet to a rangeof 0.01 to 1.0 μm in the present invention.

Because the blackening-treated layer deposited on a Zn—Ni-plated steelsheet is formed, for example, by anodic oxidation, theblackening-treated layer is not a good electric conductor. When thethickness of the blackening-treated layer is more than 1.0 μm,therefore, the electromagnetic shielding property at a seam or a jointof a case deteriorates even in the absence of a coating film. Hence, thethickness of the blackening-treated layer is set to 1.0 μm or less. Whenthe thickness of the blackening-treated layer is less than 0.01 μm, theblack appearance is poor. Hence, the thickness of the blackening-treatedlayer is set to at least 0.01 μm.

The following is a reason for limiting the arithmetical mean roughnessRa to 0.7 to 2.0 μm and the number of peaks per inch (PPI) to at least180 in the surface roughness of the blackening-treated Zn—Ni-platedsteel sheet.

Existing common blackening-treated Zn—Ni-plated steel sheets have an Rain the range of about 0.5 to 1.5 μm and the number of peaks per inch(PPI) in the range of about 120 to 160.

A blackening-treated Zn—Ni-plated steel sheet according to the presentinvention has a higher Ra, because a coating film formed on the steelsheet locally has a reduced thickness in order that a conductive regioncan be formed on the steel sheet. When the Ra is less than 0.7 μm, bumpsand dips on a surface of the steel sheet become smaller. It is thereforedifficult to form a local thin film region in the coating film. Hence,the surface roughness of the blackening-treated Zn—Ni-plated steel sheetis set to 0.7 μm or more as measured as Ra.

When the Ra is more than 2.0 μm, while the electromagnetic shieldingproperty is improved, the coating weight of coating film must beincreased to completely cover the surface of the blackening-treatedZn—Ni-plated steel sheet to exhibit corrosion resistance. This increasesthe cost. Hence, the surface roughness of the blackening-treatedZn—Ni-plated steel sheet is set to 2.0 μm or less as measured as Ra.

A blackening-treated Zn—Ni-plated steel sheet according to the presentinvention has a higher PPI in order that a large number of local thinfilm regions can be formed over the entire surface of the coating filmdeposited on the blackening-treated Zn—Ni-plated steel sheet. When thePPI is less than 180, the number of local thin film regions is small andtherefore sufficient electromagnetic shielding property is hardlyachieved. Hence, the surface roughness of the blackening-treatedZn—Ni-plated steel sheet is set to at least 180 as measured as PPI. Morepreferably, the PPI is at least 200.

While there is no limitation on the PPI, the PPI more than 400 may lowerthe corrosion resistance because of a larger number of local thin filmregions. Hence, the PPI is suitably set to 400 or less.

The thickness of a Zn—Ni-plated layer is suitably set to about 1 to 5 μmto achieve high corrosion resistance and strong adhesion.

The Zn—Ni-plated layer is not necessarily composed of ablackening-treated layer across the thickness of the layer. As describedabove, it is sufficient to form a surface portion having a thickness of0.01 to 1.0 μm with a blackening-treated layer.

The blackening treatment may be performed by anodic electrolysis,cathodic electrolysis, alternating electrolysis, or anodic oxidation ina common treatment liquid.

As a method for controlling the thickness of a blackening-treated layer,it is effective to use a method for controlling treatment conditions,such as the type, the concentration, and the pH of an oxidizing agent,which serves as a treatment liquid, the electric current density, thetime of electrolysis, and the Coulomb density.

Then, at least one coating film is formed on a blackening-treated layerthus formed, because the blackening-treated surface has insufficientcorrosion resistance.

Such a coating film may be formed of any film that can improve thecorrosion resistance after the blackening treatment without impairmentof the black appearance. Among others, a coating film composed of achromate film and an organic film or a coating film composed of anorganic film and/or an inorganic film is advantageous.

Preferably, the coating weight of coating film is at least 0.6 g/m² toachieve sufficient corrosion resistance.

The upper limit of the coating weight of coating film depends on the Raand the PPI of the surface of a Zn—Ni-plated steel sheet having ablackening-treated layer. When the Ra is 2.0 μm and the PPI is 180, theupper limit is preferably about 2.3 g/m² or less per side of the steelsheet to achieve excellent electromagnetic shielding property.

When the Ra is 2.0 μm and the PPI is 200, the upper limit of the coatingweight of coating film is preferably about 2.5 g/m² or less per side ofthe steel sheet.

When the Ra is 2.0 μm and the PPI is 400, the coating weight of coatingfilm is preferably about 5.0 g/m² or less per side of the steel sheet.

As for the upper limit of the coating weight of coating film, thearithmetical mean roughness Ra, the number of peaks per inch (PPI), andthe coating weight of coating film applied to one side of the blackcolored steel sheet satisfy the equation (1):z≦(0.010x−0.0077)y−1.05x+2.16  (1)

wherein x denotes the arithmetical mean roughness Ra (μm), y denotes thenumber of peaks per inch (PPI), and Z denotes the coating weight ofcoating film applied to one side of the black colored steel sheet(g/m²).

When the coating weight of coating film z satisfies the equation (1),the coating weight of coating film z is appropriate for the Ra and thePPI of the Zn—Ni-plated steel sheet to form sufficient conductivepoints, thus achieving particularly excellent electromagnetic shieldingproperty. FIG. 1 illustrates the relationship between the PPI (y) andthe coating weight of coating film z (g/m²) when Ra (x)=2.0 μm issubstituted into the equation (1). The coating weight of coating film ispreferred in the range illustrated in FIG. 1.

An electromagnetic shielding member formed of a black colored steelsheet and an electromagnetic shielding case formed of black coloredsteel sheets are described below.

The electromagnetic shielding property is particularly problematic at ajuncture of surface-treated steel sheets in a case formed of thesurface-treated steel sheets. Excellent electromagnetic shieldingproperty can therefore be achieved by using black colored steel sheetsaccording to the present invention at least at the juncture. This is anelectromagnetic shielding member in which a black colored steel sheetaccording to the present invention is used in all or part of the member.

Furthermore, the entire case, including the juncture, formed of blackcolored steel sheets according to the present invention can exhibitparticularly excellent electromagnetic shielding property. This is anelectromagnetic shielding case entirely formed of black colored steelsheets according to the present invention.

Examples of a method for controlling the surface roughness of ablackening-treated Zn—Ni-plated steel sheet according to the presentinvention include a method for controlling the surface roughness of acold-rolled steel sheet for use in the Zn—Ni-plated steel sheet, amethod for controlling the surface roughness of the Zn—Ni-plated steelsheet, and a method for controlling the surface roughness of theblackening-treated steel sheet. Examples of the method for controllingthe surface roughness of a cold-rolled steel sheet for use in theZn—Ni-plated steel sheet include a method for tandem-rolling ortemper-rolling the cold-rolled steel sheet with a dull roll. The dullroll is prepared by surface machining, such as blasting, electricaldischarge machining, laser machining, or etching, of a roll of a tandemrolling mill or a temper rolling mill. The cold-rolled steel may beprocessed directly by blasting.

When a Zn—Ni-plated layer according to the present invention ismanufactured by electroplating, the plated layer is formed substantiallyalong bumps and dips on the surface of a steel sheet. When theblackening treatment is performed by electrolysis, such as anodicoxidation, a blackening-treated layer is formed substantially alongbumps and dips on the surface of a steel sheet. Thus, the surfaceroughness of a blackening-treated Zn—Ni-plated steel sheet is preferablycontrolled by a method for controlling the roughness of the steel sheetbefore these layers are formed.

The surface roughness of a Zn—Ni-plated steel sheet and ablackening-treated steel sheet can be controlled with a temper rollingmill, the roughness of which is previously adjusted.

In this case, the roughness pattern of the temper rolling mill is notcompletely transferred to a steel sheet by temper rolling. The Ra of thesteel sheet is in the range of about 40% to 50% of that of the rollsurface and the PPI of the steel sheet is about 80% of that of the rollsurface. Thus, to prepare a blackening-treated Zn—Ni-plated steel sheethaving an arithmetical mean roughness Ra in the range of 0.7 to 2.0 μmand the number of peaks per inch (PPI) of at least 180, the temperrolling mill preferably has an Ra in the range of 1.4 to, 5.0 μm and thePPI of at least 220.

The electromagnetic shielding property in the present invention isevaluated by measuring leak noise with an apparatus illustrated in FIG.2.

A 20 MHz clock 4 is placed as a source of noise in a 100 mm×100 mm×100mm aluminum case 3 formed of an aluminum sheet having a thickness of 2mm. The aluminum case 3 has an 80 mm×80 mm opening at the top surface.The opening is surrounded by a 10 mm frame 5 protruding inside. A 10mm×1 mm gasket (urethane sponge wrapped in a conductive cloth(Cu—Ni-plated fiber)) 6 is placed on the frame 5. A 100 mm×100 mm sample1 is brought into contact with the gasket 6 on the top surface of thealuminum case 3 while the surface 2 to be evaluated faces downward. Thesample 1 is under a load of 19.6 N (2 kgf). An electromagnetic waveleaking from a frame-shaped matching surface between the gasket 6 andthe sample 1 is received by a loop antenna 7 having a diameter of 30 mm,which is disposed 50 mm away from the frame 5. The electromagnetic waveis amplified by a 25 dB preamplifier 8 and is analyzed with a spectrumanalyzer (Advantest Corporation, R3162) 9.

When the present invention was used as a case material, theelectromagnetic shielding property was evaluated with a case illustratedin FIG. 3.

A commercially available ATX tower-type PC case (Owltech Corporation,OWL-PCR7) was used, while an exterior portion 10 and a lid 11 of thecase were formed of a sample. Components that meet the followingspecifications were mounted in the case to fabricate a desktop personalcomputer. Windows XP was installed to boot the computer.

Power supply: a built-in power supply was used without modification.

Motherboard: A-Open, AX4SG Max II

CPU: Intel Corporation, Pentium 4 processor 3 GHz

DDR memory: generic 250 MB×2

HDD: Hitachi Global Storage Technologies, HDS722516VLAT80 (capacity 160GB)

Optical drive: Pioneer Corporation, DVR-A08-J

EXAMPLES Example 1

Samples Nos. 1 to 13 in Table 1 were manufactured according to the stepsof an annealed cold-rolled steel sheet→temper rolling (control ofsurface roughness)→electroplated Zn—Ni coating→anodic oxidation(blackening treatment)→formation of a coating film, or an annealedcold-rolled steel sheet→blasting (control of surfaceroughness)→electroplated Zn—Ni coating→anodic oxidation (blackeningtreatment)→formation of a coating film. The samples were different inthe thickness of a blackening-treated layer and the Ra, the PPI, and thecoating weight of coating film after the blackening treatment. Thethickness of a Zn—Ni-plated layer formed by Zn—Ni electroplating was 2μm. The thickness of the blackening-treated layer was controlled by theelectrolysis time and pH in the anodic oxidation.

A predetermined coating weight of coating film was deposited by applyinga paint composition containing a metal ion, a water-soluble organicresin, a water-dispersible organic resin, a glycoluril resin, and anacid, with a bar coater and curing the paint in an oven so that thetemperature of the sheet reached 190° C. in 21 seconds.

Table 1 illustrates the surface roughness, the thickness of theblackening-treated layer, the coating weight of coating film, thecorrosion resistance of a flat portion, the black appearance, and theelectromagnetic shielding property of the samples thus manufactured.

Their characteristics were evaluated as follows.

<Surface Roughness>

The blackening-treated samples were analyzed for the surface roughnesswith a stylus roughness meter (Tokyo Seimitsu Co., Ltd.) using a stylushaving a tip curvature radius of 1 μm at a scanning speed of 0.3 mm/s.The cut-off of the arithmetical mean roughness Ra according to JIS B0601-1994 was 0.8 mm. Peaks greater than 0.635 μm were counted todetermine the number of peaks per inch (PPI).

<Thickness of Blackening-Treated Layer>

Cross sections of the blackening-treated samples were observed under atransmission electron microscope. The thickness of a blackening-treatedlayer was measured at the center of the field at a magnification of90,000 and was determined as a mean value of four fields.

<Coating Weight of Coating Film>

A mass change of a sample by the formation of a coating film wasconverted into mass per unit area.

<Corrosion Resistance of Flat Portion>

A 50 mm×100 mm sample after the formation of a coating film was sealedat the end faces and was subjected to a neutral salt spray test (JIS Z2371-2000) for 48 hours. The percentage of area in which white rustgenerated was determined. On the basis of this percentage, the corrosionresistance of a flat portion was assessed according to the followingcriteria.

Excellent: 5% or less

Good: more than 5% and 10% or less

Fair: more than 10% and 20% or less

Poor: more than 20%

<Black Appearance>

The color tone (L*) of a sample after the formation of a coating filmwas measured with a spectro-photometric color difference meter (SQ2000,Nippon Denshoku Industries Co., Ltd.) and was assessed according to thefollowing criteria.

Good: L* of 25 or less

Poor: L* of more than 25

<Electromagnetic Shielding Property>

As for the electromagnetic shielding property, noise leaking from thejoint between the surface to be evaluated and the case was measured withthe spectral analyzer in the apparatus illustrated in FIG. 2. FIGS. 4 to7 show the results.

In the evaluation of the examples and the comparative examples, a peakvalue was read at a frequency of 20 Mhz to 1000 Mhz at intervals of 20Mhz. The peak value was converted into a noise score (I) by the equation(2).I=10×log(10^(0.1d1)+10^(0.1d2)+ . . . +10^(0.1dn))  (2)

n: number of peaks

d1, d2, . . . dn: peak value

FIG. 4 illustrates a measurement of a Zn—Ni-plated steel sheet having nocoating film (Reference Example), which probably exhibits excellentelectromagnetic shielding property at a seam or a joint of a case, as areference. FIG. 5 illustrates a measurement according to Example 7. FIG.6 illustrates a measurement in the absence of a sample. FIG. 7illustrates a measurement in the absence of both a sample and the outputof an electromagnetic wave. FIG. 7 illustrates extraneous noise. Peakvalues read from FIGS. 4 to 7 were substituted into the equation (2) tocalculate Is. Peaks marked with x in FIGS. 4 to 7 were assigned toextraneous noise illustrated in FIG. 7 and were excluded from thecalculation of the equation (2).

The samples according to Examples were evaluated according to thefollowing criteria:

(I−Ib)/(Ia−Ib)>0.35: Poor

0.35≧(I−Ib)/(Ia−Ib)>0.26: Fair

0.26≧(I−Ib)/(Ia−Ib)>0.20: Good

0.20≧(I−Ib)/(Ia−Ib)>0.13: Good⁺

0.13≧(I−Ib)/(Ia−Ib)≧0: Excellent

wherein a noise score I was calculated from leak noise, a noise score Iawas calculated from FIG. 6 (no sample, in the presence ofelectromagnetic wave output), and a noise score Ib was calculated fromFIG. 7 (no sample, no electromagnetic wave output). TABLE 1 Thickness ofblackening- Amount of Corrosion Sample Ra (x) Value of treated layercovering layer resistance of Black Electromagnetic No. (μm) PPI (y) Eq.(1) (μm) (z) (g/m²) flat portion appearance shielding Remarks 1 1.0 2051.6 0.15 0.6 Good Good Excellent Example 1 2 1.0 205 1.6 0.07 1.3Excellent Good Excellent Example 2 3 1.0 205 1.6 0.3 1.3 Excellent GoodExcellent Example 3 4 0.9 380 1.7 0.15 1.3 Excellent Good ExcellentExample 4 5 1.0 180 1.5 0.15 1.3 Excellent Good Good+ Example 5 6 1.3205 1.9 0.15 1.3 Excellent Good Excellent Example 6 7 1.3 260 2.2 0.151.3 Excellent Good Excellent Example 7 8 1.3 380 2.8 0.15 1.4 ExcellentGood Excellent Example 8 9 1.8 380 4.2 0.15 1.5 Excellent Good ExcellentExample 9 10 1.8 380 4.2 0.15 3.0 Excellent Good Excellent Example 10 111.0 205 1.6 0.15 1.8 Excellent Good Good Example 11 12 1.3 380 2.8 0.153.0 Excellent Good Good Example 12 13 1.0 120 1.4 0.15 1.3 Good GoodFair Comparative Example 1 14 1.3 150 1.6 0.15 1.3 Excellent Good FairComparative Example 2 15 1.3 205 1.9 1.1 1.3 Excellent Good PoorComparative Example 3 16 1.0 205 1.6 0 0 — — Excellent Reference Example

As is apparent from Table 1, the samples according to the presentinvention, which had a blackening-treated layer having a thickness inthe range of 0.01 to 1.0 μm and had an arithmetical mean roughness Ra inthe range of 0.7 to 2.0 μm and the number of peaks per inch (PPI) of atleast 180 after blackening treatment, exhibited not only an excellentblack appearance and high corrosion resistance, but also excellentelectromagnetic shielding property.

When the arithmetical mean roughness Ra (x), the number of peaks perinch (PPI) (y), and the coating weight of coating film applied to oneside of a steel sheet (z) satisfy the equation (1), the electromagneticshielding property was particularly excellent.

Example 2

Table 2 illustrates the electromagnetic shielding property of varioussamples (Nos. 5, 6, 8, 13, and 15) illustrated in Table 1, when thesamples were applied to an exterior portion 10 and a lid 11 of thetower-type PC case illustrated in FIG. 3.

The electromagnetic shielding property of the tower-type PC case wasevaluated as follows.

<Electromagnetic Shielding Property of Electronic Equipment andElectrical Equipment>

The electromagnetic shielding property of electronic equipment andelectrical equipment including a case formed of a sample was measured inan open area test site at a distance of 3 m as described in note 2 ofTable 4.6 in technical standards V-3/2005.04, 4.3 tolerance of radiatedemission electric field strength issued by the Voluntary Control Councilfor Interference by Information Technology Equipment (VCCI). Theacceptability was determined by a Class B criterion.

Pass: satisfy Class B

Fail: does not satisfy Class B TABLE 2 Exterior Electromagnetic Case No.portion Lid shielding Remarks 1 No. 5 No. 6 Pass Example 13 2 No. 6 No.6 Pass Example 14 3 No. 8 No. 6 Pass Example 15 4 No. 13 No. 6 FailComparative Example 4 5 No. 15 No. 6 Fail Comparative Example 5

As is shown in Table 2, when the exterior portion and the lid of thecase were formed of a black colored steel sheet according to the presentinvention (cases Nos. 1 to 3), the case had satisfactory electromagneticshielding property.

1. A black colored steel sheet having an excellent electromagneticshielding property, comprising a Zn—Ni-plated steel sheet having ablackening-treated layer; and a coating film formed on the Zn—Ni-platedsteel sheet, wherein the blackening-treated layer has a thickness of0.01 to 1.0 μm; the Zn—Ni-plated steel sheet having theblackening-treated layer has an arithmetical mean roughness Ra of 0.7 to2.0 μm and the number of peaks per inch (PPI) of at least
 180. 2. Theblack colored steel sheet according to claim 1, wherein the number ofpeaks per inch (PPI) is 200 to
 400. 3. The black colored steel sheetaccording to claim 1, wherein the arithmetical mean roughness Ra, thenumber of peaks per inch (PPI), and a coating weight of coating filmapplied to one side of the black colored steel sheet satisfy theequation (1):z≦(0.010x−0.0077)y−1.05x+2.16  (1) wherein x denotes the arithmeticalmean roughness Ra (μm), y denotes the number of peaks per inch (PPI),and Z denotes the coating weight of coating film applied to one side ofthe black colored steel sheet (g/m²).
 4. An electromagnetic shieldingmember, wherein all or part of the member is formed of the black coloredsteel sheet according to claim
 1. 5. An electromagnetic shielding caseformed of the black colored steel sheet according to claim
 1. 6. Theblack colored steel sheet according to claim 2, wherein the arithmeticalmean roughness Ra, the number of peaks per inch (PPI), and a coatingweight of coating film applied to one side of the black colored steelsheet satisfy the equation (1):z≦(0.010x−0.0077)y−1.05x+2.16  (1) wherein x denotes the arithmeticalmean roughness Ra (μm), y denotes the number of peaks per inch (PPI),and Z denotes the coating weight of coating film applied to one side ofthe black colored steel sheet (g/m²).
 7. An electromagnetic shieldingmember, wherein all or part of the member is formed of the black coloredsteel sheet according to claim
 2. 8. An electromagnetic shieldingmember, wherein all or part of the member is formed of the black coloredsteel sheet according to claim
 3. 9. An electromagnetic shieldingmember, wherein all or part of the member is formed of the black coloredsteel sheet according to claim
 6. 10. An electromagnetic shielding caseformed of the black colored steel sheet according to claim
 2. 11. Anelectromagnetic shielding case formed of the black colored steel sheetaccording to claim
 3. 12. An electromagnetic shielding case formed ofthe black colored steel sheet according to claim 6.